The invention relates to a method of generating a visual appearance of different gray level in a liquid crystal display screen having a passive matrix of pixels incapable of generating dots which are really grey but have only an active or an inactive state for each pixel, a surface having an apparent grey level being obtained by activating a given proportion of pixels, and the screen being virtually divided into pixel blocks all having the same dimension, while patterns of dots are defined in advance, each pattern representing a block and corresponding to one of the grey levels due to the fact that it has a number of active dots corresponding to this grey level, a pixel contained in a block of the image being displayed with the active or the inactive state of the dot which, in a pattern corresponding to the grey level desired for said pixel, has the same position as the pixel in the block comprising this pixel.
The invention also relates to a device for generating the appearance of grey levels in a liquid crystal display screen having a passive matrix of pixels incapable of generating really grey dots, comprising a wired logic circuit which computes the state in which a pixel is to be displayed as a function of its position in a block and as a function of the grey level desired for this pixel.
The invention is used in devices for displaying stationary images, notably in videophones, "Minitel", or portable computers.
A method and a device for generating different grey levels in a liquid crystal display screen is known from the document DE 39 06 924. In the system described in this document the screen is divided into blocks each comprising a plurality of pixels and a desired grey level is obtained by activating a certain proportion of pixels of a block as a function of the desired grey level, and the choice of the position of the pixels which are activated is modified in the course of time by a cyclic shift of the position of the active pixels within a block. Since the shift of the active dots is relatively regular, it is to be feared that other troublesome visual effects appear which are different from those eliminated by means of the method described in this document. Moreover, the circuit proposed for carrying the method into effect is relatively complex and comprises a large number of logic elements.